The present invention relates to a device, typically incorporated in a sport shoe, which prevents or at least minimizes dorsiflexion of a user's foot relative to the user's leg beyond a predetermined optimum angle while permitting further forward flexion of the leg relative to the sport shoe performing surface. The invention is particularly well suited for use while skiing in downhill ski boots but is also usable for other sport shoes where limiting dorsiflexion to some optimum angle during the sport performance is desired.
A sport shoe forms the connection between an athlete and the surface on which he or she performs, such as the ski and mountain slope for skiing, the playing field for such sports as soccer, football or tennis, or the road or path along which a runner runs. Major maneuvers of the athlete require the transmission of forces between the runner's leg and the ground via the sport shoe. These maneuvers are accompanied by conscious immovement or movement of the athletes ankle, that is, muscular activity to immobilize or mobilize the foot relative to the leg. Compared to other major body joints there is weak muscular control and limited range of motion of the foot in dorsiflexion.
To enable the sport shoe to efficiently transmit often significant forces, the sport shoe must provide the proper support for the ankle. At the same time, the sport shoe must be designed so that it allows the athlete to perform all necessary ankle movements and make the most efficient use of his or her muscular strength when performing such movements.
Although this general description of the function of a sport shoe applies to use in virtually all sports, the degree of movement and the magnitude of force to be applied by the lower extremity to execute various maneuvers are particularly evident in downhill skiing. As a consequence, of all the sport shoes, downhill ski boots are the most elaborate. Briefly, a downhill ski boot provides an exterior shell for the foot and an exterior cuff for the leg which extends well above the ankle. Such boots permit a forward and rearward flexion of the leg with respect to the foot from a preselected "normal" position or dorsiflexion and plantarflexion of the foot relative to the leg, respectively, but they prevent significant medial and lateral or adduction and abduction movements of the foot with respect to the leg, i.e. in all other directions the entire boot is relatively rigid. In the past, this has been accomplished by constructing downhill ski boots of a two-part, substantially rigid shoe defined by a lower foot section and an upper leg section that is typically pivotally attached to the lower foot section. In the interior of the shell is a relatively soft liner. In use, the boot and in particular the sole, which forms part of the lower foot section, is engaged by a binding attached to the ski to thereby rigidly connect the boot to the ski.
While skiing, the boot tightly encompasses the athlete's foot and leg, typically by means of one or more buckles which tighten the boot against the foot and the lower leg. Because of the many gross movements and the exertion of large forces during many turning maneuvers executed by a downhill skier, the boot must be relatively tight on the foot and leg. Frequently, the required tightness is uncomfortable, can reduce blood circulation, and can lead to pain and fatigue. Any looseness of the boot, on the other hand, greatly compromises the athlete's ability to maneuver the skis because of the poor transmission of forces from the leg to the skis.
To overcome this problem, the applicant has previously invented ski boots having dynamic fitting systems disclosed in the above referenced patent applications. Such fitting systems allow a relatively snug and comfortable fit of the boot on the athlete's leg. However, the fit is momentarily tightened in response to relative movement of the leg, typically between his or her foot and leg. Normally, this is accomplished by providing an instep strap, a movable footbed, an adjustable tongue, or the like, which are operatively connected with the lower shell and the upper cuff so that upon relative movement between them, the tightness of the fit of the boot increases proportionally to the extent to which the upper cuff moves relative to the lower shell away from a "normal" position. In ski boots, the "normal" position of the upper cuff typically includes some degree of forward angulation of the upper cuff with respect to the lower shell. Any additional forward flexion of the lower leg increases the tightness of the fit. Upon return of the upper cuff to its normal position, the tightness of the fit lessens.
Actual tests with such boots have shown that they constitute a remarkable improvement over conventional ski boots which lack a dynamic fitting system. Specifically, discomfort, pain, poor circulation and fatigue which often accompanied prior art ski boots have been substantially eliminated. The tight fit required for executing turning maneuvers and the like during skiing is attained during the turning maneuver. At all other times the fit is less tight and more comfortable.
In spite of the significant improvement provided by the dynamic fitting systems discussed above, sport shoes in general and ski boots fitted with such systems in particular can be improved. Specifically, such dynamic fitting systems affect the tightness of the fit as soon as there is any movement between the lower shell and the upper cuff. This, applicant has discovered, is not always desirable because it is essential that ski boots, for example, provide for an adequate range of motion for the ankle joint in certain skiing conditions. This range of motion allows the foot and shoe to provide a stable platform when the athlete makes subtle changes in the center of gravity of his or her body. An adequate range of ankle motion is also highly desirable to accommodate the finer muscle movements which take place during certain piloting maneuvers in skiing. In other sports such as soccer, basketball and tennis, sudden stops and starts, rapid accelerations and quick changes in direction while performing in these sports demand that the ankle have this mobility to assure center of gravity stability and muscular control for the athlete.
Yet, the sport shoe should also enable the athlete to most advantageously utilize his or her maximum muscle strength. Most maneuvers requiring great strength occur in dorsiflexion. In skiing, for example, major changes in direction involve the efficient muscular control of the foot in dorsiflexion for the effective shift in the center of gravity, anticipation, angulation and edging. To obtain the optimum muscular control of the ankle in this posture of dorsiflexion there is a particular position that must be attained and retained from which the various strength related maneuvers can be executed. This position is referred to as the optimum dorsiflexion angle. The existence of an optimum dorsiflexion angle can be traced to certain observed physiologic characteristics of muscle and the anatomical orientation of the flexor and extensor muscles of the leg and foot. Among the several characteristics of muscle that must be considered are the following:
(1) muscle mass strength is greatest when the muscle is near its greatest length (Kreighbaum, et al., Biomechanics, A Qualitative Approach for Studying Human Movement, Burgess Publishing Co., at pp. 123, 124);
(2) muscle mass strength decreases with increased velocity of contraction (Piscopo and Baley, Kinesiology, The Science of Movement, John Wiley & Sons, at pp. 150-151); and
(3) muscle mass strength is dependent upon the angle of pull against the boney lever arm (Cooper, et al., Kinesiology, The C. V. Mosby Co., at pp. 116-123).
In addition, muscle mass strength is greatest when there is no contraction (Cooper, et al., Kinesiology, The C. V. Mosby Co., at p. 109).
Applicant has discovered that optimum strength for skiing maneuvers is attained when the relative angular inclination between the foot and the leg, i.e. dorsiflexion, is approximately 12.degree.. The 12.degree. dorsiflexion angle, however, does not provide proper body balance or positioning of the center of gravity during all phases of skiing. In downhill skiing, when leaving the fall line, often a greater forward flexion of the leg relative to the ski is required than the optimum dorsiflexion angle. This forward flexion is necessary to resist the sideslip of the ski caused by the curved trajectory and pull of gravity. During this drive down the fall line, as the edge angle is increased, the ski becomes more resistant to sideslip, develops an increasing reverse camber and holds better at the tip and tail. The arc of the turn, the rate of movement, and the closeness to the fall line determines the angulation and therefore forward flexion of the leg required to resist the sideslip caused by the centrifugal force. Yet, prior art dynamic fitting systems incorporating a movable footbed maintained a given angularity between the footbed and the cuff. If that angularity is chosen for optimum efficiency, e.g. at 12.degree. proper balance will not be attained much of the time. On the other hand, if the relative forward angulation of the cuff relative to the footbed is chosen at a lesser value, say between 7.degree. to 9.degree. forward angulation as is typical, optimum strength cannot be attained.
From the foregoing, it is apparent that there is a present need for an improved dynamic fitting system which includes a movable footbed that is constructed so as to provide some freedom of motion for the ankle joint without tightening the fit. Further, there is a present need for a dynamic fitting system in which the relative angular inclination between the foot and the leg is such as to provide comfort for the athlete, and which readjusts the relative angular inclination during times when maximum strength is required and allows further forward flexion of the leg relative to the shoe performing surface, so as to enable the athlete to exert the greatest possible force at that instance.